The present invention relates to an arrangement for controlling the pressure in brake actuating cylinders of a vehicle, and more particularly to an arrangement of this type which is usable in a hydraulic braking system of a motor vehicle provided with an antiskid control equipment, and with an auxiliary source of pressurized hydraulic fluid.
There are already known pressure controlling arrangements of this type which include a master cylinder device comprising a housing defining a bore, at least one master piston in the bore, and at least one feeding port in the housing. A conventional arrangement of this type further includes a pedal-actuatable brake-valve device located in front of the master cylinder device and operative for the introduction of auxiliary energy from an auxiliary energy supply system including a hydraulic reservoir, a pump, a pressure accumulator, as well as associated connecting conduits, electromagnetically actuatable control valves in the brake conduit connected to the master cylinder device for the purpose of brake anti-skid control of the wheel brake actuating cylinders having a return conduit to the hydraulic reservoir, wherein a hydraulic closing device is inserted in the return conduit and is normally open and closes upon failure of the auxiliary energy, and a hydraulic conduit between the outlet of the brake-valve device and at least one replenishment chamber associated with the master cylinder device and communicating with the associated feeding port.
A brake system equipped with anti-skid control device and constructed in this manner is known from German published patent application DE-OS No. 24 43 545. In this construction, the outlet of the brake-valve device directly communicates with the two replenishment chambers of the master-cylinder device which is designed as a tandem master cylinder, and, when the brake pedal is depressed, auxiliary energy is directly supplied to the secondary side and from there to the operating side of the sleeve-sealed master cylinder pistons in order to dynamically actuate the wheel brake cylinders of both brake circuits. The tandem master cylinder is designed as a stepped piston having at its end close to the brake pedal an enlarged flange portion of a greater diameter than the piston itself, so that a pressure chamber is formed during operation on the secondary side of the first piston which is close to the pedal, this pressure chamber being adapted to be fed with auxiliary energy, and being in communication with the associated replenishment chamber of the respective piston. In the presence of auxiliary energy, the pressure in the pressure chamber urges the piston against an axial stop rigid with the housing in opposition to the pedal-force direction, so that, during normal braking operation, the piston is kept stationary in the presence of auxiliary energy. Upon failure of the auxiliary energy, the stepped piston coupled to the pedal-actuatable brake-valve device will be mechanically actuated which results in a static actuation of the wheel brake cylinders. In this arrangement, the hydraulic closing device in the return line, which is normally open, will be closed so that no pressure fluid is able to escape from the brake circuits. Thus, the known brake antiskid control apparatus is usually operated by auxiliary energy, and only in case of emergency, when the auxiliary energy fails, will the stepped piston operate in the manner which is conventional in the static actuation of the brake circuits. Due to the continuous introduction of auxiliary energy during operation, a large quantity of auxiliary energy has to be kept constantly available; the auxiliary energy system is required to be dimensioned in a correspondingly voluminous manner. In addition to the consumption of a large quantity of auxiliary energy resulting in a considerable power loss of the device, it is furthermore a disadvantage of this conventional arrangement that fail-free braking operation is not ensured in all instances upon failure of the auxiliary energy supply system, that is, the arrangement may become inoperative due to possible leaks in the sleeve seal of the individual master-cylinder pistons which cannot be perceived during normal operation of the brake with the auxiliary source in operation.